Most people ITT are idiots.
AV-Comparatives - Independent Tests of Anti-Virus Software
Avast is one of the much better AV's out there, and I used to use it a lot. Its free too, but does bother you from time to time about upgrades and shit. Gaming mode disables mostly all of it. NOD doesnt slow your system down too much, but it does have a really tight grip on it. That tends to happen when you have an AV designed to be as thorough as NOD is. NOD is definitely one of the best 3 you can get, along side G-data and Kapersky. You are pretty much gonna have to pay for kapersky though, as I dont think I ever found a crack for it. Never tried G-data.
Avira is right along side of Avast, so it really comes down to preference.
Of course, if you have minimal competence in computers, (ie, not a fucking retard) MSE does more than enough to suit you just fine. Its fast, free, very lightweight, and has no premium only features. Combine it with Malwarebytes, and if you wind up getting a virus, that means you are an idiot. Rootkits are a different beast altogether and usually require more specialty tools like tdsskiller.